Cospeciation (1 of 3)
A species of louse lives on a species of gopher. When the gophers get together to mate, the lice get an opportunity to switch gophers and perhaps mate with lice on another gopher. Gopher-switching allows genes to flow through the louse species.
Cospeciation (2 of 3)
When the gopher lineage splits into lineages A and B, lice have few opportunities for gopher-switching, and lice on gopher lineage A don't mate with lice living on gopher lineage B. This "geographic" isolation of the louse lineages may cause them to become reproductively isolated as well, and hence, separate species.